Vertical gate nand memory devices

ABSTRACT

In an example, a device comprises a vertical stack of memory cells. Each memory cell of the vertical stack may include more than one memory element. A first vertical gate line may be coupled to a first one of the memory elements in each memory cell, and a second vertical gate line may be coupled to a second one of the memory elements in each memory cell. The first vertical gate line may be electrically isolated from the second vertical gate line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional of U.S. Provisional Application No. 61/844,555 filed on Jul. 10, 2013 entitled VERTICAL GATE NAND MEMORY DEVICES and is incorporated by reference herein in its entirety.

COPYRIGHT NOTICE

© 2013 Rambus, Inc. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all rights whatsoever available under 37 CFR § 1.71(d).

BACKGROUND

In order to address scalability issues associated with planar NAND flash, vertical NAND flash structures have been developed. In one known vertical NAND flash structure, the NAND gate is run vertically to achieve a relatively small cell footprint. However, some applications demand an even smaller cell footprint for a given amount of memory than provided by the known vertical NAND flash structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified cross-section diagram showing a vertical gate NAND memory cell.

FIG. 1B is a circuit diagram showing the vertical gate NAND memory cell of FIG. 1A.

FIG. 2 is a simplified cross-section diagram showing a vertical memory stack including multiple vertical gate NAND memory cells shown in FIG. 1A.

FIG. 3A is a perspective simplified cross-section diagram showing a stacked memory device including multiple vertical memory stacks shown in FIG. 2.

FIG. 3B is a circuit diagram showing the stacked memory device of FIG. 3A.

FIG. 4 is a circuit diagram showing memory access operations for a selected string of a stacked memory device represented, e.g., by the configuration of FIG. 3A.

FIG. 5A is a block diagram showing a stacked memory device with isolated horizontal channel tracks.

FIG. 5B is a simplified cross-section diagram showing one of the FIG. 5A vertical gate NAND memory cells with two isolated memory devices.

FIG. 5C is a perspective partial device diagram showing memory access operations for one string pair of the stacked memory device with isolated horizontal channel tracks of FIG. 5A.

FIGS. 6A-61 are block diagrams showing an example process for fabricating the vertical gate NAND memory cell with isolated horizontal channel tracks.

FIG. 7 is a flow chart illustrating an example process for fabricating the vertical gate NAND memory cell with isolated horizontal channel tracks.

DETAILED DESCRIPTION

FIG. 1A is a simplified cross-section diagram showing a vertical gate NAND memory cell.

The vertical gate NAND memory cell 10 includes a first vertical gate line 11 electrically coupled to an upper word line 14, and a second vertical gate line 12 electrically coupled to a lower horizontal word line 15 that is electrically isolated from the upper horizontal word line 14.

The vertical gate NAND memory cell includes more than one NAND memory element, e.g. two separately accessible NAND memory elements as per the illustrated example. A first one of the separately accessible NAND memory elements comprises a portion of the charge trap layer 16 and a portion of the doped polysilicon layer 18. A second one of the NAND memory elements comprises a portion of the charge trap layer 17 and a portion of the doped polysilicon layer 18.

FIG. 1B is a circuit diagram showing the vertical gate NAND memory cell of FIG. 1A. The circuit diagram shows a gate terminal corresponding to the first NAND memory element electrically coupled to the vertical gate line 11, and a gate terminal corresponding to the second NAND memory element electrically coupled to the vertical gate line 12.

FIG. 2 is a simplified cross-section diagram showing a vertical memory stack including multiple vertical gate NAND memory cells shown in FIG. 1A.

The vertical memory stack 20 comprises a plurality of vertical gate NAND memory cells. In the illustrated example, the plurality of vertical gate NAND memory cells comprises four vertical gate NAND memory cells for ease of illustration; however, it should be understood that any number of memory cells may be included in a stack. In the stacked memory device, the gate lines 21 and 22 are connected to the word lines 24 and 25 by the vias 28 and 29, respectively.

FIG. 3A is a perspective simplified cross-section diagram showing a stacked memory device including multiple vertical memory stacks shown in FIG. 2. The stacked memory device 30 includes multiple of the vertical memory stacks 20 of FIG. 2. FIG. 3B is a circuit diagram showing a portion of the stacked memory device of FIG. 3A.

FIG. 4 is a circuit diagram showing memory access operations for a selected string of a stacked memory device represented, e.g., by the configuration of FIG. 3A.

The circuit diagram illustrates a memory access to data in one layer of a vertical memory stack, the data located in a horizontal string of tandem memory cells crossing the same layer in a horizontally adjacent line of vertical memory stacks. During a memory access operation on the memory string, any unselected memory cells of stacks other than the selected stack in the memory string have their respective memory elements turned on by voltages on vertical gate lines coupled to the unselected memory cells. A selected memory cell in the selected stack in the memory string is configured to have an unselected memory element turned off and a selected memory element set to a memory access voltage by voltages on vertical gate lines coupled to the respective memory elements of the selected memory cell.

FIG. 5A is a perspective partial device diagram showing a stacked memory device with isolated horizontal channel tracks. The stacked memory device 50 includes vertical stacks of memory cells that each include more than one memory element, and a horizontal insulation structure 69 to electrically isolate the memory elements. FIG. 5B is a simplified cross-section diagram showing one of the FIG. 5A vertical gate NAND memory cells with two isolated memory devices.

As with prior embodiments, the stacked memory device 50 includes vertical gate lines, each coupled to one memory element in each of the two memory cells. In an example, a first vertical gate line is coupled to a first one of the memory elements in each of three vertically aligned memory cells, and a second vertical gate line is coupled to a second one of the memory elements in each of the same three memory cells. The first vertical gate line is electrically isolated from the second vertical gate line.

In an example, a first word line (upper word line) is positioned above the vertical stack of memory cells and horizontally perpendicular to the vertical gate lines, and a second word line (lower word line) is positioned below the vertical stack of memory cells and horizontally perpendicular to the vertical gate lines. The first word line is electrically coupled to a first one of the vertical gate lines, and the second word line is electrically coupled to a second one of the vertical gate lines.

FIG. 5B is a block diagram showing vertical gate NAND memory cell with two isolated memory devices shown in FIG. 5A.

In an example, the memory elements in each of the memory cells are configured to store respective data states. In an example, a first one of the memory elements in each of the memory cells is separately accessible from a second one the memory elements in each of the memory cells. In an example, an insulator 69 is used to electrically isolate the memory elements within a cell.

FIG. 5C is a circuit diagram showing memory access operations for one string pair of the stacked memory device with isolated horizontal channel tracks of FIG. 5A.

The circuit diagram illustrates a memory access to data in one layer of a vertical memory stack of a string of tandem memory cells crossing the same layer in a horizontally adjacent line of vertical memory stacks. During a memory access operation on the memory string, memory elements electrically coupled to the lower word lines are turned off. Unselected memory elements electrically coupled to the upper word lines are turned on by voltages on vertical gate lines coupled thereto. A selected memory element of the memory string is set to a memory access voltage by a voltage on a selected upper word line and a vertical gate coupled thereto.

FIGS. 6A-61 are block diagrams showing an example process for fabricating the vertical gate NAND memory cell with isolated horizontal channel tracks.

Referring to FIG. 6A, a first insulator type, e.g. an oxide, may be deposited, and a second insulator type that is different than the first insulator type, e.g. a nitride, may be deposited above a layer of the first insulator type. The process may be repeated any number of times. In an example, the deposition provides a thin film having the first insulation layers interleaved with the second insulation layers.

Referring to FIG. 6B, any known process may be used to remove material from the deposited layers in order to form vertical insulation structures having first insulation layers interleaved with second insulation layers. In an example, a trench etch on the thin film is used to form the vertical insulation structures.

Referring to FIG. 6C, recess channels may be formed in the vertical insulation structures by removing a portion of the second insulator type material. In an example, removal is by performing a recess etch of the vertical insulation structures corresponding to the second insulation layers to form the recess channels in the insulation structures.

Referring to FIG. 6D, a semiconductive material may be deposited in the recess channels. In an example, polysilicon is deposited between the vertical insulation structures.

Referring to FIG. 6E, the semiconductive material, e.g. the polysilicon, between the vertical insulation structures may be removed while retaining the semiconductive material, e.g. the polysilicon, in the recess channels. In an example, an anisotropic polysilicon etch is performed to remove the polysilicon from between the vertical insulation structures and to retain the polysilicon in the recess channels.

Referring to FIG. 6F, a charge-trap layer may be deposited over the semiconductive material in the recess channels to form vertical stacks of memory elements, each memory element comprising a combination of semiconductive material in one of the recess channels and a portion of the charge-trap layer. In an example, a charge-trap thin film may be deposited over side walls between the vertical insulation structures to form the vertical stacks of memory elements. In an example, the charge-trap layer is an oxide nitride oxide layer, and the memory elements are silicon oxide nitride oxide silicon (SONOS) memory devices.

Referring to FIGS. 6G-I, vertical gate structures may be formed in the channels between the vertical stacks of memory elements. In an example, a semiconductive material may be deposited over the charge-trap layer between adjacent vertical insulation structures. In an example, a trench etch may be formed on the semiconductive material to form the vertical gate structures.

FIG. 7 is a flow chart illustrating an example process for fabricating the vertical gate NAND memory cell with isolated horizontal channel tracks.

In block 701, vertical insulation structures may be formed from a thin film having the first insulation layers interleaved with the second insulation layers. In block 702, recess channels may be formed in the vertical insulation structures corresponding to the second insulation layers. In block 703, a semiconductive material may be deposited in the recess channels.

In block 704, a charge-trap layer may be formed over the semiconductive material in the recess channels to form vertical stacks of memory elements. In block 705, vertical gate structures may be formed in channels between the vertical stacks of memory elements.

One of skill in the art will recognize that the concepts taught herein can be tailored to a particular application in many other ways. In particular, those skilled in the art will recognize that the illustrated examples are but one of many alternative implementations that will become apparent upon reading this disclosure.

Although the specification may refer to “an”, “one”, “another”, or “some” example(s) in several locations, this does not necessarily mean that each such reference is to the same example(s), or that the feature only applies to a single example. 

1. A device, comprising: a vertical stack of memory cells, each memory cell including more than one memory element; a first vertical gate line coupled to a first one of the memory elements in each memory cell; and a second vertical gate line coupled to a second one of the memory elements in each memory cell, wherein the first vertical gate line is electrically isolated from the second vertical gate line.
 2. The device of claim 1, wherein the memory elements in each of the memory cells are configured to store different data states.
 3. The device of claim 1, wherein the first one of the memory elements in each of the memory cells is separately accessible from the second on the memory elements in each of the memory cells.
 4. The device of claim 1 further comprising: a first word line positioned above the first vertical stack of memory cells and perpendicular to the first vertical gate line, wherein the first word line is electrically coupled to the first vertical gate line; and a second word line positioned below the vertical stack of memory cells and perpendicular to the second vertical gate line, wherein the second word line is electrically coupled to the second vertical gate line.
 5. The device of claim 1, further comprising an array of vertical stacks including and similar to said vertical stack, wherein the memory cells in the array are configured to form memory strings, each comprising electrically connected memory cells from a row of vertical stacks of memory cells, and wherein, during a memory access operation on a selected one of the memory strings, any unselected memory cells in the selected memory string have their respective memory elements turned on by voltages on vertical gate lines coupled to the unselected memory cells.
 6. The device of claim 5, wherein, during the memory access operation on the selected memory string, a selected memory cell in the memory string is configured to have an unselected memory element turned off and a selected memory element set to a memory access voltage by voltages on vertical gate lines coupled to the respective memory elements of the selected memory cell.
 7. A method comprising: forming recess channels in vertical insulation structures having first insulation layers interleaved with second insulation layers; depositing a semiconductive material in the recess channels; and depositing a charge-trap layer over the semiconductive material in the recess channels to form vertical stacks of memory elements, each memory element comprising a combination of semiconductive material in one of the recess channels and a portion of the charge-trap layer.
 8. The method of claim 7, wherein forming the recess channels further comprises performing a recess etch of the vertical insulation structures corresponding to the second insulation layers to form the recess channels in the insulation structures.
 9. The method of claim 7, further comprising forming the vertical insulation structures from a thin film structure having the first insulation layers interleaved with the second insulation layers.
 10. The method of claim 9, wherein the forming of the vertical insulation structures further comprises performing a trench etch on the thin film to form the vertical insulation structures.
 11. The method of claim 7, wherein depositing semiconductive material in the recess channels comprises: depositing polysilicon between the vertical insulation structures; and performing an anisotropic polysilicon etch to remove the polysilicon from between the vertical insulation structures and to retain the polysilicon in the recess channels.
 12. The method of claim 7, wherein depositing the charge-trap layer comprises depositing a charge-trap thin film over side walls between the vertical insulation structures to form the vertical stacks of memory elements.
 13. The method of claim 7, wherein the charge-trap layer is an oxide nitride oxide layer, and wherein the memory elements are silicon oxide nitride oxide silicon (SONOS) memory devices.
 14. The method of claim 7 further comprising forming vertical gate structures in the channels between the vertical stacks of memory elements.
 15. The method of claim 14, wherein forming the vertical gate structures comprises: depositing semiconductive material over the charge-trap layer between adjacent vertical insulation structures; and performing a trench etch on the semiconductive material to form the vertical gate structures.
 16. A device comprising: a vertical stack of memory cells, each memory cell including more than one memory element and a horizontal insulation structure to electrically isolate the memory elements; and vertical gate lines, each coupled to corresponding ones of the memory elements in each memory cell.
 17. The device of claim 16, wherein the vertical gate lines comprise first and second vertical gate lines electrically isolated from each other.
 18. The device of claim 17 further comprising: a first word line positioned above the vertical stack of memory cells, wherein the first word line is electrically coupled to the first vertical gate line; and a second word line positioned below the vertical stack of memory cells, wherein the second word line is electrically coupled to the second vertical gate line.
 19. The device of claim 16, wherein the memory elements in each of the memory cells are configured to store respective data states.
 20. The device of claim 16, wherein a first one of the memory elements in each of the memory cells is separately accessible from a second one the memory elements in each of the memory cells. 